The invention relates to a low phonon energy gain medium and to active devices comprising such a gain medium.
Optical gain media are well-known as forming the basis of both lasers and optical amplifiers. Many gain media are solid-state and comprise a host material doped with active dopant ions, such as rare earth ions. Silica is a particularly common host material for amplifiers, partly because the material is widely used in optical fibers. A silica amplifier is highly compatible with silica transmission fiber; they can be coupled together with low losses.
An integrated fiber laser may be fabricated from such a gain medium by additionally including a photosensitive dopant such as tin. The photosensitivity of tin allows optical gratings to be written directly into the fibers to provide the necessary cavity mirrors. Co-doping of silica fibers with the rare earth ions erbium or ytterbium for gain, and tin for photosensitivity, has been used to produce fiber lasers [1]. Tin-oxide-doped silica glass has been produced by modified chemical vapor deposition (MCVD) and the sol-gel technique [2-4], and tin-oxide-doped silica glass ceramics by the sol-gel technique alone [3, 4].
An important characteristic of a host material for a gain medium, is its principal optical phonon energy. A large phonon energy is associated with short carrier lifetimes arising from large non-radiative recombination rates. For a host material having a maximum phonon energy Ep and an active dopant having a lasing energy transition ΔE, the nonradiative recombination rate is a very strong function of the ratio R=ΔE/Ep. The lower the value of R, the higher is the probability of undesirable nonradiative recombination. Therefore, host materials having low phonon energies Ep are required, especially for small lasing energy transitions ΔE.
Unfortunately, silica has a high principal optical phonon energy, which arises from contributions from vibrations of the Si—O bond, in the vicinity of 1000 cm−1 [5]. Therefore, although silica is a useful host material as regards its compatibility with commonly used fibers, it is disadvantageous as far as phonon energy is concerned.
Fluoride glasses have been proposed as an alternative to silica [5, 6]. The phonon energy is 500 cm−1, which makes these materials more suitable for hosting active dopants with a low ΔE. However, fluoride glasses have low compatibility with silica optical fibers, and it is difficult to fabricate low loss fibers directly from fluoride glasses.